1. Field of the Invention
The present invention relates to an optical switch for switching paths between input channel waveguides and output channel waveguides, utilizing the deflection of optical beams.
2. Description of the Related Art
In recent years, the transmission band of optical communication has been enlarged, and the speeding up and large capacity thereof has been developed combined with the wavelength division multiplexing (WDM) technology. In order to construct hardware infrastructures of optical fiber network in a backbone communication network, an optical switch for switching transmission determinations of optical signals is required.
As a conventional optical switch, there has been known an optical switch utilizing a movable micro-mirror. Further, in order to further develop the high integration and high-speed of the optical switch, the development of optical switches utilizing a change in the refractive index due to an electro-optic effect of ferroelectric substance has been progressed. As one of the optical switches utilizing this electro-optic effect, there is the one having a configuration in which an optical deflection element is disposed between an input port and a plurality of output ports, and a traveling direction of a light which is given to the input port is deflected by the optical deflection element, thereby guiding the light to the desired output port. As a specific example, there has been proposed an optical switch for deflecting a light by a prism type domain reversal optical deflection element or a prism type electrode optical deflection element, utilizing a lithium niobate (LiNbO3) single crystal wafer on which titanium (Ti) diffusion type waveguides or proton exchange type waveguides are formed (refer to the literature: “Guided-Wave Electro-Optic Beam Deflector Using Domain Reversal in LiTaO3” by Q. Chen et al., Journal of Lightwave Tech., Vol. 12, pp. 1401–1404, 1994).
As a specific configuration of the optical switch utilizing the electro-optic effect as described above, as shown in FIG. 6 for example, there has been known the one comprising an optical input section 110, a collimate section 120, a first optical deflecting section 130, a slab type optical waveguide section 140, a second optical deflecting section 150, an optical condensing section 160 and an optical output section 170 (refer to Japanese Unexamined Patent Publication No. 2000-180905 and the literature: “Electro-Optic Beam Deflection Switch for Photonic Burst Switching” by A. Sugama et al., ECOC 2004, No. 4.6.3 (2004)). In the optical switch of FIG. 6, a light input to each of input channel waveguides Cin1 to Cin4 in the optical input section 110 is converted into a parallel light by each of collimate lenses 121 corresponding to the input channel waveguides Cin1 to Cin4, in the collimate section 120, and thereafter, is deflected by the first optical deflecting section 130 including prism type electrodes 131, according to a position of a desired one of output channel waveguides Cout1 to Cout4, to be transmitted through the slab type optical waveguide section 140 toward the second optical deflecting section 150. Then, the light incident on the second optical deflecting section 150 including prism type electrodes 151 is again deflected, and thereafter, is condensed on one point by a condenser lens 161 corresponding to the desired output channel waveguide, in the optical condensing section 160, to be guided to the desired one of the output channel waveguides Cout1 to Cout4.
Further, typically, since optical signals transmitted through an optical fiber have random polarization planes, an optical switch used for an optical communication system is required to be polarization independence. In order to meet this requirement, as shown in FIG. 7 for example, there has been proposed an optical switch in which a half-wave plate 141 is disposed on the center of the slab type optical waveguide section 140 in the conventional configuration example shown in FIG. 6 (refer to Japanese Unexamined Patent Publication No. 2003-280053). In the optical switch shown in FIG. 7, a polarization plane of a light being propagated through the slab type optical waveguide section 140 is rotated by 90° by the half-wave plate 141. As a result, a TE mode and a TM mode are converted into each other, and the deviation on the time base between the TE mode and the TM mode, which occurs on the former half of the slab type optical waveguide section 140, is offset on the latter half of the slab type optical waveguide section 140, so that polarization mode dispersion (PMD) occurring within the optical switch is compensated.
According to the conventional optical switch utilizing the electro-optic effect as described above, by using a material having the high electro-optic effect for the first and second optical deflecting sections 130 and 150, it becomes possible to reduce drive voltages applied to the prism type electrodes 131 and 151, and to achieve an increase of deflection angle in each of the first and second optical deflecting sections 130 and 150. However, in the case where the material having the high electro-optic effect is used, since the polarization dependence of the deflection angle becomes prominent, there is a problem in that a loss of the light which is guided to the desired output channel waveguide is increased.
Here, there will be described in detail the polarization dependence of the deflection angle.
A deflection angle of an optical deflection element utilizing the electro-optic effect depends on a change in the refractive index due to the electro-optic effect. To be specific, when the electric field E generated by the prism type electrode is added to the material having the electro-optic effect, refractive index “n” of the material is changed. A change Δn of this refractive index can be expressed in the following formula (1), if a primary electro-optic effect (Pockels effect) acts.Δn=−½·n3·r·E  (1)In the above formula, “r” is an electro-optic constant.
In the case of using orientations of large electro-optic constant, it is typically known that electro-optic constant “r” and refractive index “n” of the ferroelectric substance each has the polarization dependence in each orientation. For example, in the case where LiNbO3 of z-cut is used as the electro-optic material, electro-optic constant r33 corresponding to the TM mode is 0.8 pm/V and electro-optic constant r13 corresponding to the TE mode is 8.6 pm/V, and also, refractive index nTm corresponding to the TM mode is 2.14 and refractive index nTE corresponding to the TE mode is 2.2.
In the optical switch of the configuration as shown in FIG. 6, in the case where the ferroelectric substance such as LiNbO3 or the like is used as the material of the first and second optical deflecting sections 130 and 150, due to the polarization dependence of electro-optic constant “r” and of refractive index “n” as described above, as shown in FIG. 8 for example, the deviation occurs between the deflection angle to the TE mode light (solid line) and the deflection angle to the TM mode light (broken line). Due to this deviation between the deflection angles, the TM mode light in the light given to the input channel waveguide Cin1 reaches a position deviated from the desired output channel waveguide Cout3. Therefore, a loss of the light which is guided from the input channel waveguide Cin1 to the output channel waveguide Cout3 is increased.
Further, in the optical switch in which the half-wave plate 141 is disposed on the center of the slab type optical waveguide section 140 as shown in FIG. 7, in the case where the ferroelectric substance is used as the material of the first and second optical deflecting sections 130 and 150, although the polarization dependence on the time base can be resolved by the half-wave plate 141, it is impossible to compensate for the deviation between the deflection angles based on the polarization dependence of electro-optic constant “r” and of refractive index “n”, as shown in FIG. 9 for example. Therefore, similarly to the case shown in FIG. 8, the loss of the light which is guided to the desired output channel waveguide is increased.
Incidentally, in the conventional optical switch shown in FIG. 6 or FIG. 7, since the electro-optic material without the polarization dependence has been basically used as the material of the first and second optical deflecting sections 130 and 150, the polarization dependence of the deflection angle as described above has not been problematic. To be specific, it is known that PLZT of epitaxial growth (Pb1-xLax(ZryTi1-y)1-x/4O3) has the polarization independence depending on the composition thereof (refer to the literature: “Epitaxial PLZT Waveguide Technologies for Integrated Photonics” by K. Nashimoto, Integrated Photonics Devices, Materials, and Technologies IX, Photonics West 2005 (Invited paper)). Such an electro-optic material with the polarization independence has been applied to the conventional optical switch. However, the material with the polarization independence such as PLZT of epitaxial growth has the electro-optic constant smaller than that of an electro-optic material with the polarization dependence (for example, bulk SBN (Sr0.75Ba0.25Nb2O6)). In order to further reduce the drive voltage for the optical switch and to realize the larger deflection angle, it is desired to configure the first and second optical deflecting sections 130 and 150 using the electro-optic material with the polarization dependence as described above.